1. Field of the Invention
This invention pertains to a process of making glycidyl ethers of phenolic compounds by the reaction of epichlorohydrin with the phenols. More particularly, this invention pertains to a process for making low molecular weight liquid epoxy resins from the reaction of epichlorohydrin with polyhydric phenols.
2. Prior Art
The prior art is replete with techniques of preparing glycidyl ethers of phenols and particularly for preparing glycidyl ethers of polyhydric phenols.
The usual preparation of glycidyl ethers of phenols involves the reaction of epichlorohydrin with a phenol in the presence of an alkali metal hydroxide, as represented by the following equation: ##STR1## The stoichiometry of the reaction requires one equivalent of epichorohydrin and one equivalent of base per phenolic hydroxyl group. However, excessive amounts of undesirable by-products are produced if one uses a stoichiometric ratio of reactants. To maximize the yield of the glycidyl ether, excess epichlorohydrin and excess base have been used to a substantial advantage. One of the major problems associated with the use of excess epichlorohydrin has been the recovery of unreacted epichlorohydrin from the reaction product.
A typical multistep method for preparing glycidyl ethers of phenols which results in the production of low molecular weight epoxy resins is described in U.S. Pat. No. 2,943,095 (Farnham et al.). Farnham et al. conducted the reaction of epichlorohydrin with a polyhydric phenol in two steps: In the first step, the phenol is reacted with excess epichlorohydrin in the presence of a base (e.g., lithium chloride, lithium acetate and alkali metal hydroxides) to give a product comprising the propylene chlorohydrin of the phenol. This reaction is generally referred to as the coupling reaction. Farnham et al. report that in stage one, the reaction temperature should not exceed 45.degree. C. In step two, the coupled reaction product is dehydrohalogenated with a strong base to form the corresponding glycidyl ether of the phenol. This dehydrohalogenation is normally performed in a mixture of liquids comprising a volatile water-soluble alcohol or ketone and a water-immiscible liquid. Farnham et al. recover excess epichlorohydrin by distillation between steps one and two.
Many other patents have issued which also use a multistep reaction and which teach that various catalysts may be used to promote the coupling reaction.
Reinking et al. (U.S. Pat. No. 2,943,096) indicate that the coupling step is greatly facilitated by conducting the reaction under anhydrous conditions in the presence of a quaternary ammonium salt catalyst. This reference teaches that the coupling reaction should be conducted at temperatures less than 60.degree. C. At temperatures in excess of 60.degree. C., undesirable polymeric compounds are formed.
Others have utilized sulfonium salts (e.g., British Pat. No. 1,019,565), tertiary amines or quaternary ammonium salts as catalysts for the coupling reaction (e.g., British Pat. No. 897,744) and still others have utilized phosphonium salts (German AUS. No. 2,335,199) and/or phosphoranes (e.g., German Offen. No. 2,533,505). The latter two German references are the closest prior art relative to step one in the instant process in that these references do at least use catalysts containing phosphorus.
The dehydrochlorination step has also been the subject of investigation. Smith (U.S. Pat. No. 3,372,442) teaches that the dehydrochlorination is facilitated by using an aqueous sodium hydroxide solution which is substantially saturated with sodium carbonate. Becker (U.S. Pat. No. 3,980,679) teaches that the dehydrochlorination is facilitated by adding solid alkali metal hydroxide incrementally to the reaction mixture.
None of the aforementioned references teach that there is any advantage in conducting the dehydrochlorination step in the presence of a quaternary onium salt nor do they indicate what effect, if any, the residual quaternary onium salts or other catalysts have on the reaction product or on this particular step of the overall reaction. It is known, however, that amines and quaternary ammonium salts are capable of causing degradation of the epoxides. And at least one of the references specifically teaches that the catalyst for the coupled reaction should be removed from the reaction mixture prior to the dehydrochlorination reaction (British Pat. No. 897,744). The other references are not as explicit in their written description but convey the same teaching in that they wash the coupling reaction product with water to remove the catalyst and/or water-soluble by-products, (e.g., sodium chloride and other salts) before the dehydrochlorination step. Still other references handle the problem by passing the glycidyl ether of the phenol through various clays or other materials to absorb the residual catalyst from the reaction product.
The glycidyl ethers of phenols can be prepared according to the above techniques using either batch processes or by continuous processes. This is illustrated by the above references and by German Pat. No. 2,522,745 and German Pat. No. 2,523,696.